The present invention relates generally to an image forming apparatus, and more particularly, to a control system and method for an intermediate transfer member of an image forming apparatus in which a surface wheel and attached encoder is used to directly measure and control the motion quality of the transfer member.
Color electrophotographic (EP) printers are commonly utilized to form an image on a recording sheet or other tangible medium. In color electrophotography, an image is created on the surface of an imaging member composed of a photoconducting material, by first uniformly charging the surface, and then selectively exposing areas of the surface to a light beam. A difference in electrostatic charge density is created between those areas on the surface which are exposed to the light and those areas that are not exposed to the light. The latent electrostatic image is developed into a visible image by electrostatic toners, which are selectively attracted to either the exposed or unexposed portions of the photoconductor surface, depending on the relative electrostatic charges on the photoconductor surface, the development electrode and the toner. Toners of various colors may be applied to the electrostatic images in order to produce different color planes. After toning, each color plane is transferred to a transfer media, at an image transfer station.
Color EP printers are typically one of two types. The first type of printer is a revolver type in which a transfer media makes multiple passes past a single image transfer station, receiving a separate color plane from the imaging member during each pass. Alternatively, the printer may be of the tandem type, in which a transfer media makes a single pass by multiple image transfer stations, accumulating and superposing color planes from each station during the pass. Both types of printers include an intermediate transfer member (ITM), such as a transfer belt, which may serve as the transfer media. Color planes from each of the transfer stations may be accumulated on the transfer belt with a subsequent, single transfer to a tangible media, such as paper. Alternatively, the transfer belt may be used to transport a paper sheet or other tangible media past the image transfer station(s), so that the color planes are accumulated directly on the paper.
Because color tandem EP printers superpose color planes from multiple transfer stations to form a single, multi-color image, they are susceptible to print quality defects that arise from misregistration of the color planes that are successively deposited on the accumulating media. In order to reduce the misregistration errors due to the color planes being transferred at different spatial positions, each of the transfer station positions must be known or predicted with great precision (e.g.,  less than 50 um), so that successive color planes can be registered acceptably for print quality. However, many sources of error are inherent in an ITM mechanical subassembly that can create errors of 50 um or more. For instance, when an ITM belt is driven by a constant speed motor at one of a plurality of belt rollers, belt velocity errors may arise from: 1) runout of the drive roller, 2) belt thickness variations (which affect the effective diameter of the drive roller), and/or 3) tension variations in the belt (which may be different at each color plane transfer point). The integration of belt velocity between color transfer stations determines the position error. Other position errors may also arise independent of velocity and relate to the path followed by a belt of varying thickness over rollers that have varying amounts of runout.
A number of attempts have been made to characterize the ITM mechanical subassembly during the run-in or calibration cycle of the printer itself, in order to reduce misregistration between the color planes. These characterization attempts have included generating and transferring a test pattern from each imaging member onto the belt, using a complex sensor to detect the test pattern position on the belt to an accuracy of better than 50 um, and correcting the belt speed or position based upon the internal calibration. While these characterization procedures have reduced misregistration errors, and thereby improved print quality, such processes are costly, waste toner, consume machine time at each calibration (e.g. 2 minutes), and add significant complexity to the machine.
In a color EP machine, the ITM belt is typically driven by a motor shaft, which rotates a drive roller through a gear reduction. To control the speed of the drive roller, and thus the velocity of the transfer belt, prior motion control systems have relied upon feedback coming directly from the motor shaft to control the drive motor. However, depending upon the quality of the gear reduction and the quality of the drive roller, the feedback from the motor shaft may not accurately represent the true velocity or position of the transfer belt. Thus, even with the motor shaft feedback, the resulting motion quality of the ITM may be relatively poor. The poor motion quality of the ITM can result in poor color plane registration and poor overall print quality.
Accordingly, to reduce misregistration errors between superposed color planes and improve print quality, it is desirable to have a motion quality control system for an ITM that accurately reflects the true motion of the ITM belt. Further, it is desirable to have such a motion quality control system that eliminates errors associated with drive roller eccentricities, transfer belt thickness variations, and other velocity/position signatures of the belt subassembly without the complexity, time and toner waste associated with characterization procedures.
Accordingly, it is an object of the present invention to provide an improved motion quality control for an intermediate transfer member in an image forming apparatus.
In particular, it is a primary object of the present invention to provide a system and method for controlling the velocity of an intermediate transfer member in a printer, in which the surface motion of the transfer member is directly measured and fed back to a transfer member drive motor in order to maintain a constant velocity for the transfer member. By directly measuring the surface motion of the intermediate transfer member, and providing the measured motion as a feedback signal to the intermediate transfer member drive motor, the drive motor is able to react directly to changes in the surface motion of the transfer member belt. Thus, a constant velocity may be maintained for the intermediate transfer member without the need to characterize the transfer belt during the run-in or calibration cycle of the printer.
To achieve the foregoing and other objects, and in accordance with a first aspect of the present invention, a motion control system for controlling the motion of an intermediate transfer member in an image forming apparatus is provided in which a measuring member directly measures the surface motion of the intermediate transfer member and generates signals proportional to the velocity of the member. The measured surface motion is provided as a feedback signal to a motor controller, which compares the signal with a reference signal. The difference between the signals is used to adjust the control of a drive motor for the intermediate transfer member drive roller in order to maintain a constant velocity for the intermediate transfer member.
In accordance with a second aspect of the present invention, a method of controlling the motion of an intermediate transfer member in an image forming apparatus is provided which includes the steps of directly measuring the surface motion of the intermediate transfer member, providing the measured surface motion as a feedback signal to a motor controller for the intermediate transfer member, and adjusting the velocity of the intermediate transfer member in accordance with the feedback signal.
In accordance with a third aspect of the present invention, a color image forming apparatus for forming an image by superposing a plurality of color planes on a transfer media is provided which includes one or more image forming members for forming a plurality of different color toner images and an intermediate transfer member for receiving each of the different color toner images at a transfer point. A drive member rotates the intermediate transfer member, while the surface motion of the member is directly measured by a measuring member. The measured surface motion is provided as a feedback signal to a controller for the drive motor of the intermediate transfer member drive roller, in order to control the velocity of the intermediate transfer member in accordance with the measured motion.
Still other objects and advantages of the present invention will become apparent to those skilled in this art from the following description and drawings, wherein there is described and shown a preferred embodiment of this invention in one of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of other different embodiments, and its several details are capable of modification in various, obvious aspects all without departing from the scope of the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive.